In a couple of commercially available dental materials (meth)acrylate based monomers are used.
Sometimes a comparably high molecular weight difunctional (meth)acrylate based monomer like e.g. Bis-GMA is mixed with a comparably low molecular weight (meth)acrylate based co-monomer like e.g. triethyleneglycol dimethacrylate (TEGDMA) having also a comparably low viscosity.
Especially the comparably high molecular weight monomer sometimes shows a structural principle of having a comparably stiff hydrocarbon backbone in the centre of the monomer like e.g. a bisphenol A or a tricyclodecane moiety. (Meth)acrylate functional groups are attached to this backbone as (meth)acrylic acid esters via an aliphatic chain that may contain hetero atoms like e.g. oxygen. These materials typically show on average a volume shrinkage of about 2.0 to about 3.5 vol-% during curing determined according to the Archimedes buoyancy principle, DIN 13907 correlated with Linometer measurements (cf. Am. J. Dent. 2000, 13 (special issue), 82D-84D and Dent. Mater. 1993, 9, 11-14).
High volume shrinkage during curing might cause unwanted and/or detrimental side-effects like e.g. enamel fracture, cracked cusps, and cuspal movement as well as failures of the composite-tooth interface and thus, should be reduced. On the other hand dental compositions should have sufficient mechanical properties.
In order to maintain a high level of mechanical properties of the cured composition but also to reduce the volume shrinkage during curing, different approaches have been suggested.
One typical approach is to raise the amount of inorganic filler. This, however, may cause a lower degree of conversion during curing of the (meth)acrylate groups because of the reduced amount of organic resin present.
Another approach is to increase the molecular weight of the monomers. In this respect, using pre-polymeric (meth)acrylate functional urethanes based on bisphenol derivatives have been suggested.
A further approach is to use a mixture of different difunctional (meth)acrylate monomers in a special ratio or a mixture of special monofunctional and difunctional (meth)acrylate monomers.
However, due to the complex reaction behaviour of multi-component compositions, a lot of effort is needed to achieve sometimes only marginal improvements of volume shrinkage, if a sufficient level of mechanical properties is to be maintained.
U.S. Pat. No. 4,744,827 describes (meth)acrylic acid derivatives of tricyclodecanes and their use.
US 2004/0209990 refers to a low shrinking polymerizable dental material comprising a mixture of organic or inorganic filler and a polymerizable resin matrix having a volumetric polymerization shrinkage of less than 2 vol.-%.
WO 2005/107626 relates to dimer acid-derived dimethacrylates and their use in dental restorative compositions.
JP 2306955 is focussing 2,2-bis(4″-hydroxyphenyl)hexafluoropropane and/or its polyalkylene glycol esters. It is stated that these molecules can be useful as crosslinkers in coatings, adhesives, inks, electrical insulators, optical materials, photographical materials, printing, dental cements, fibers, photoresists, pharmaceutical products etc. However, it has been found that bisphenol A derivatives containing CF3 moieties sometimes show a comparable low refractive index and are not always suitable in the dental field.
From the above it becomes clear that there is still room for improvement, especially with regard to the requirements to be fulfilled with respect to modern dental materials.